The present invention relates generally to telephone data transmission systems, and more particularly to a dial-in data transmission system which allows a central computer, for example, to access a remote data set, such as an electronic publication storage device, a television polling system, a computer data base subscriber data set, or a utility meter or plurality of utility meters located in a business or residence, via non-dedicated telephone lines and communicate with that remote data set without substantially interfering with normal use of the subscriber's telephone. Even more particularly, the present invention relates to dial-in data transmission systems wherein the data device to be accessed dials up a central computer via telephone lines to communicate with the computer at a selected time and stands by to detect calls from the central computer at other selected times.
The present invention finds application in many areas, one of which, for instance, might be a shop by computer system where it might be necessary to transmit information to and from an individual subscriber's data set connected to an information storage device. Another might be the field of utility usage data retrieval, wherein a central computer accesses a remote data set which has stored therein data corresponding to the electrical or gas or other utility usage at the remote location. Thus, the present invention is useful for allowing a utility to read a utility meter located at a remote location, for example an electric meter or a gas or water usage meter. Still another application might be remote control of heating, ventilating and air-conditioning equipment through telephone lines, as for instance, in a utility's load management program. Although the invention will be described for use with telephone lines, it will be apparent to one skilled in the art that the invention can also be used with transmission media other than telephone lines.
Various systems are presently known which allow central computers to access individual subscriber's data sets via non-dedicated telephone lines. For example, see U.S. Pat. Nos. 4,126,762, 4,104,486, 4,394,540 and 4,345,113. In the 4,104,486 and 4,394,540 patents, the telephone is isolated from the telephone line during a selected time window. In U.S. Pat. No. 4,126,762, in one embodiment, a time window is utilized. In another embodiment, sequential ringing signals, having a different time spacing than signals from a normal telephone call condition a remote terminal to intercept the ringing signals and to provide selective connection of a data accumulator to the telephone line. It is preferable to use the subscriber's existing telephone line rather than install a separate dedicated line to provide access to the data set. Thus, systems of this type require that the telephone line be available for use both by ordinary persons and by computers calling the subscriber. Requirements of systems of this type are that when computer calls are received, the call is automatically transferred to a data set and the telephone preferably should not ring.
In applicant's prior U.S. Pat. No. 4,345,113, an automatic telephone message intereption system is described which permits the use of a single telephone line for both ordinary personal use and for receiving calls from a computer, wherein computer calls are automatically transferred to the subscriber's data set without ringing the subscriber's telephone. An ordinary (non-computer) call made to the subscriber, would, however, be answered by the automatic telephone message interception system described in this U.S. patent even if no one were present at the subscriber's telephone. The caller would thus be charged for a brief call even though no actual person answered the phone.
The subscriber could, of course, solve this problem by shutting off the automatic system if the telephone is to be left unattended, but the data set could not be accessed by the computer while the system was shut off.
The need thus existed for a system which both allows a central computer to access the subscriber's data set via a non-dedicated telephone line and which allows normal operation of the telephone by both the subscriber and other persons calling the subscriber. In particular, such a system should allow a computer to access the subscriber's data set without ringing the subscriber's telephone and should allow persons to call the subscriber at any time without being charged for the call if the telephone is unanswered by a person.
Applicant's prior U.S. Pat. No. 4,469,917, issued Sept. 4, 1984, describes an apparatus which satisfies the above need.
In U.S. Pat. No. 4,469,917, a system and apparatus is described for connecting a central computer to a remote subscriber's data set using the subscriber's non-dedicated telephone line during a preselected window in time. 27 Furthermore, in applicant's prior U.S. Pat. No. 4,578,534, the apparatus described in U.S. Pat. No. 4,469,917 was modified to permit connection of the central computer at any time to a remote subscriber's data set using the subscriber's non-dedicated telephone line and without interference of any sort with normal use of the subscriber's telephone other than suppression of the first ring of an incoming call.
In the system of U.S. Pat. No. 4,469,917, the apparatus therein described detects a ringing signal on the telephone line during a preselected window in time during which the subscriber's data set is connected to the telephone line and the subscriber's telephone instrument is disconnected. The apparatus does nothing until the ringing signal stops, indicating that the caller has hung up. The apparatus then causes the data set to dial up the central computer and proceed to transmit or receive data during the time window.
In the device described in applicant's prior U.S. Pat. No. 4,578,534, the telephone instrument is normally disconnected from the telephone line. The central computer issues a ringing signal preferably having a single ring burst, and a device described in that patent detects the single ring burst and triggers a data set to dial-up the central computer and provide data over the telephone line to the central computer. If a ringing signal with more than one ring burst is detected, then the device recognizes that the call is not from the central computer and disconnects the data set from the telephone line and connects the telephone instrument to the telephone line, enabling the telephone to ring and the user to answer the call. Because the telephone is normally disconnected, if a ringing signal with only one ring burst is detected, the telephone subscriber never hears the call from the central computer.
In a further development, applicant's prior U.S. Pat. No. 4,654,868, the apparatus described in U.S. Pat. No. 4,578,534 was modified in order to allow its use in applications where more than one data set or data device is located at the remote location. For example, in U.S. Pat. No. 4,654,868, an apparatus is described where a plurality of consumption level devices, for example, electricity, gas and water meters, are interrogated at the remote location by a central computer
Accordingly, the above patents describe various apparatus for allowing the central computer to access a remote data set via, for example, telephone lines. In applicant's estimation, the simplest system for allowing a central computer to access a remote data set, for example, to read a utility meter, is the so-called "dial-in" system wherein the meter interface device (data set) is connected directly across the telephone line, and a clock in the interface device causes the interface to dial the central computer's telephone number at a particular time each month and transmit the meter reading to the computer. Applicant's prior U.S. Pat. No. 4,469,917 describes a modified example of this type of device, improved to prevent interference with the subscriber's use of the telephone device at any time, even when the computer is communicating with the data set, and which allows a computer first to call the data set during a time window with the data set not answering but subsequently calling the computer back (in contrast to the simple dial-in system wherein the data set merely calls the central computer at the preselected time), and wherein a clock is utilized to disconnect the telephone instrument from the transmission line during predetermined time intervals allowing the computer to call the data set with the data set not answering but subsequently calling the central computer to provide communication during that time interval.
The major disadvantage of time-synchronized dial-in systems is the inability of the computer to establish contact with the data set at any time other than that programmed by the clock. Thus, for example, if a house or business is sold and the seller's utility account must be closed out, a meter reader must be dispatched to take a final reading on the day of the sale if that day does not coincide with the programmed time. This can be a time-consuming and expensive task, especially considering that in many areas of the country, 20% of the houses change hands each year. Accordingly, it would be advantageous if a system could be provided which would allow a central computer access to a remote data set not only at the preselected time (for example, once a month), but also during a preselected interval in time as required by the needs of the central computer, for example, a utility required to read a utility meter on a particular day.